The work proposed uses heterocyst formation in Anabaena 7120 as a model system for the identification and characterization of the genes and, consequently, the molecular mechanisms that organisms use to determine cell fate. Elucidation of the controls of cellular differentiation is critical to understanding the loss of cellular growth control that causes a perturbation of the differentiated state of the cell, which can lead to a diseased condition such as neoplastic cell growth and cancer in humans. Filaments of Anabaena 7120 are easily cultured, have a short generation time, are genetically manipulable, and can be induced to form a predictable pattern of two distinct cell types. Recently, several genes have been isolated that can distort this pattern. Two genes in particular, hetR and patA appear to control the differentiation and patterning of heterocysts along a filament of vegetative cells. Extra copies of hetR cause innappropriate excessive differentiation of vegetative cells to heterocysts, whereas lack of hetR prevents heterocyst formation. The molecular functions of HetR and PatA are unknown, as is control of their production. Two novel genetic selections will be used to separately isolate genes whose products act upstream and downstream of HetR/PatA, and biochemical methods will be used to elucidate the functions and the relationship between HetR and PatA. By dissecting the hetR/patA regulatory cascade, we hope to elucidate a control circuit that will be instructive for questions of cellular differentiation in both bacteria and multicellular organisms and the diseases that can result from a disturbance in the control of cell fate determination.